Concentration of caustic



April 9 1940- l. E. MusKA-r 2.196.594

CONCENTRATION oF oAUsTIc Filed nec. 11. 1937 :s sheets-sheet 1 A 9,1940. l. E. MusKAT 2,196,594

CONCENTRATION 0F CUSTIC Filed Dec. 11. 1937 3 Sheets-Sheet 2 .MEL

April 9, 1940.

l. E. MUSKAT CONCENTRATION 0F CAUSTIC Filed nec. 11. 1937 s sheets-sheets Patented Apr. 9, 1940 PATENT I OFFICE CONCENTRATION F CAUSTIC 'IrvingE. Muskat, Akron, Ohio, assignor to Pitts' burgh Plate Glass Company,vAllegheny County, Pa., a corporation o! Pennsylvania ApplicationDecember 11, 1937, Serial No. 179,334

24 Claims.

This invention relates to the concentration,

purlncation and crystallization of certain solutions of alkali metalhydroxides and more particularly to the dehydration, crystallization,and purification, (partial or complete), of aqueous solutions of sodiumhydroxide of at least 5Q percent concentration.

This application is a continuation-in-part of my copending application93,022, filed July 28, 1936.

Caustic soda solutions, as initially produced in commercial processes,are relatively dilute solutions, the concentrations of which are ingeneral from 8 to 12 percent by weight. These solutions are-contaminatedwith a large number of impurities, such as sodium sulphate, sodiumchloride, chlorates, iron, etc., which must be removed in order toproduce a commercially satisfactory product.

In the neighborhood of about a 50 percent concentration of caustic soda,many of these impurities have a minimum solubility in caustic solutions,and since the production of a concentrated caustic from these relativelydilute solutions is necessary, the usual practice is to concentrate theimpure caustic to this concentration by simple vaporization and toremove these impurities by settling or ltering. If greater purity isdesired, crystallization of sodium hydroxide according to Patent No.1,733,897 to Hooker and Marsh, the 50 percent ltered caustic liquor isdiluted to approximately 39 percent, at which concentration the 3?/2hydrate is caused to crystallize at a reduced temperature of 10 C.Approximately 50 percent of the caustic is crystallized. The motherliquor which contains the impurities cannot be satisfactorily purifiedand is sold as an impure product. The 31/2 hydrate crystals are thenmelted and reconcentrated to 50 percent.

These various manipulations entail such a large expenditure of time,labor, and money that electrolytic caustic, which must be purified bysuch a process, as been at a decided disadvantage in compe ing withcaustic made by the ammonia-soda process. Even after purifyingelectrolytic caustic by means of these expensive crystallizationprocesses, the final puried product is more corrosive than lime-sodacaustic.

The preparation of a purified 50 percent caustic, however, is not theonly difficulty. It is commonly recognized that caustic soda in higherconcentrations is corrosive, and is readily contaminated. Hence, everymanipulation'through as a hydrate may be resorted to. For example,

which the caustic is put, after it has been purifled, results incontamination by reason of the fact that the caustic attacks theapparatus in which it is treated. Often it has been considered desirableto dehydrate the 50 percent caustic to produce '10 to 75 percent, oreven anhydrous, caustic. The production of '70 percent caustic isusually carried out by vaporization at reduced pressures. Even with thebest of equipment, the heated product corrodes the apparatus and becomescontaminated and thus the production of a '70 percent caustic, havingapurity equal to that of the initial 50 percent caustic from which it wasproduced, has not been satisfactorily accomplished by any commercialprocess of which I am aware.

The purity oi the product is even further reduced when concentrationfrom 'I0 percent to the anhydrous state is attempted. Since evaporation,as carried out with dilute caustic liquors, is no longer possible,vthecaustic is placed in cast iron pots and vigorously heated in an openflame. In order to drive o the last traces of water, temperatures ashigh as 1100 F. must be resorted to. Again the corrosive nature of thecaustic at these high temperatures is so great that it attacks the potextensively and further contamination of the caustic takes place.

Sulfur is added to precipitate the iron and even after completedehydration, the caustic must be kept molten for many hours to allow theprecipitate to settleout. Even with these operations, commercialpreparation of pure anhydrous caustic has been well nigh impossible. Sofar as I am aware, there is no known process of commercially producinganhydrous caustic of a purity which even approximates that of a 50percent solution.

Numerous other methods of concentration have been proposed, but nonehave been accepted commercially. Among these is a method described in U.S. Patent 1,961,590, granted to R. B. Mac- Mullin, wherein it isproposed to dehydrate aqueous caustic solutions of 20 to 30 percentconcentration, by addition of 125-200 parts of liquid ammonia to 100parts of such caustic solutions, the result being the formation of twoAliquid layers. Upon separation of the lower layer and elimination ofammonia by evaporation, a more concentrated caustic solution isproduced; however, if it is attempted to produce a substantially greaterconcentration, the yields are relatively 'low since a very substantialproportion of the initial caustic (in general, upwards of 35-40%)migrates into the upper ammonia layer. This v results in the productionofsuch a large quantity of. a caustic solution which is more dilute thanAthe initial solution 4being treated that the `process described by thispatent can not be used solution does not result in a substantially moreconcentrated solution, nor are the yields increased;V and in some casesno separation or concentration takes place. For example, when 8, and 20parts'of ammonia, respectively, are added to 1 part of 20 to 30 percentcaustic soda solution, no separationv into different liquid layers isobtained and a quantity of solid material precipitates out of solution;while when 400 parts of liquidammonia are added to 100 parts of suchcaustic solution, two layers are formed, but. only about percentv'of thecaustic is `separated, as a solution of substantially percentconcentration. This yield is too low to be commercially signicant.Further reduction of the ratio of ammonia increases the yield butdecreases the concentration of the caustic.

In addition, the process as described by this patent is not directed to,nor adapted for the removal of impurities and avoidance of contaminationsince, at the low concentrations of caustic soda dealt with herein,satisfactory removal of such impurities as sodium chloride can not beobtained by liquid ammonia treatment.

inthe order of about percent or higher; or if required, above 69 percentconcentration, or even an anhydrous product. At the same time, I am ableto obtain a further puri'cation of the caustic soda liquor by removal ofvarious salts,` in

y particular, sodium chloride and sodium chlorate,

notwithstanding the presence of more or less water in the ammonia.

The term liquid ammonia in this application is intended to includeanhydrous liquid ammonia and mixtures of ammonia and water which containat least enough ammonia (in general above 65 percent NH3 concentration)so as to insure the separation of a liquid phase consistingpreponderantly of sodium hydroxide or sodium.

hydroxide and water, when the ammonia and caustic are brought together.The mixture of ammonia and water described above is often designated inthis specification as partially hydrated liquid ammonia. i

There is found .to be a marked difference in the phase' relationshipswhich are established when high concentrations of caustic soda, for

example, 50 percent or above, are treated withl liquid ammonia, andthose which arise when more dilute solutions, such as 20 to 30 percentsolutions of caustic soda, are so treated. In the treatment of about 50percent and higher concentrations, caustic containing substantially morechloride, chlorates, and others, are dissolved by escaso( than 50percent sodium hydroxide may be produced and it is possible thereby toobtain any desiredy concentration from 50 percent to,the A anhydrousstate.

In the separation of phases that takes place 5 when such solutions aretreated, a solid phase may also be separated in greater or lessproportion in the presence' of a concentrated caustic liquid phase andan ammonia phase.. This solid differs in caustic concentration from the'caustic liquor and is in general, of greater causticv content. Hence,by regulating the quantity of solid withdrawn with the concentratedcaustic liquor, very high concentration of caustic may be obtained andthe concentration of caustic maybe easily controlled. This regulation isnot feasible in treatment of 20-30 percent solutions since upon additionof the enormous quantitiestof ammonia required to cause precipitation ofsubstantial quantities of solid from such solutions, no appreciableseparation yof a concentrated caustic liquor from an ammonia liquor canbe detected. In addition, the yields obtained by thev treatmentof 50percent solution and above are extremely high and only very smallquantities of the' .initial NaOH (usually not more than 4 or 5 percent)`moves into the upper layer. These yields markedly distinguish from the25 to 50 percent yields obtainable in the treatment of 20 to 30 percentsolutions heretofore described. Furthermore, the addition of 8 to 20parts of anhydrous liquid ammonia tofone part by weight of 50 percentcaustic results in a separation of liquid phases, whereas no suchseparation is obtainable when such proportions of liquid ammonia tocaustic are added to 20 to 30 percent solutions as heretofore set forth.Additionally, the treatment of caustic soda of 50 percent concentration,or above, with liquid ammonia results in a removal of certainimpurities, particularly, sodium chloride, to a degree which is whollyunattainable when 20 to 30 percent-solutions are so treated. I havefound that 50 percent caustic can be concentrated to any desired extentup to the anhydrous state by subjecting the caustic to progressivetreatment by repeated washing or extraction with liquid ammonia.Theseextrac tions may be eilected by repeatedly washing the caustic withanhydrous liquid ammonia, or partially hydrated liquid ammonia, as forexample, by treating the caustic with progressively e increasingconcentrations of liquid ammonia. In

each case, each washing is followed by a removal of part or all of theammonia liquonbefore the '65 the ammonia phase and are removed to alarge extent. In this way, my process both avoids the contaminationduring concentration which has been the objectionable feature of priorart processes, and in fact also eil'ects a further puriflca- 70 tion.Hence, by my processi am able to secure highly concentrated causticwinch is just as pure or even purer than 50 percent solutions, as abovereferred to and by combining my process with a caustic purificationprocess, I am able to obtain u atada results commercially which hithertowere regarded as impossible.

A `method of treatment in'accordance 4vwith my invention which I `havefound to be highly emcient, comprises concentrating and purifying thehydroxide solution by `permitting it to settle or otherwise pass througha body or plurality of bodies of liquid ammonia. I may conduct such aprocess, for example, by introducing the solution to be concentratedinthe upper portion of a column and allowing it to settle thru a risingbody of liquid amonia'which is introduced intol a lower portion of thecolumn. 'I'he concentrated caustic is withdrawn from the lower part ofthecolumn,

while the ammonia, together with the absorbed water, is withdrawn fromthe upper portion thereof.

The invention will be fully understood by reference to the followingdescription illustrated by the accompanying drawings in which- Fig. 1 isa diagrammatic sectional view of apparatus suitable for carrying out themethod embodying my invention. y

Fig-2 is a diagrammatic sectional view illustrating a modification ofFig. 1.

Fig. 3 is a ow sheet diagrammatically illustrating another suitablemethod for carrying out the process.

The equipment used in carrying out the-present invention is ofsulciently sturdy construction to withstand the pressures necessary tocarry out the process. The apparatus may be of steel, or any othersuitable material. In the specific form illustrated in Fig. 1,hereinafter described, I may use a jacketed cylinder I0, having a bottomI2, preferably conical, connected to a conduit I4 which discharges intoa receiving receptacle I6. The temperature in the cylinder I0 can becontrolled either by means of the steam jacket I I, or by means ofcoils, electrical heating elements, or any other suitable means. Causticsoda solution to be concentratedis admitted to the cylinder III somewhatbelow the top thereof, through a conduit I8, and liquid ammonia isadmitted at a point above the bottom of the cylinder through a conduit20. The ammonia liquor, after it has become admixed with water andimpurities carried by the caustic, finds exit through a dischargeconduit 22 at the top of the cylinder, and may pass directly to anammonia still (not shown), for reconcentration to be subsequentlyreused, or may be employed for purification as hereinafter described.

'Ihe cylinder I0 may be provided with an agitator, which suitablycomprises a suitably journaled shaft 24 extending coaxially of thecylinder and supported at one end by a bearing secured in spider 30. Theother end of the shaft extends outwardly through a gland (not shown) inthe top of the cylinder where it is provided with suitable drivingmeans, such as a pulley 32. The shaft carries suitable transverselyextending mixing or agitating blades 26 which terminate at a substantialdistance above the bottom of the shell I0, thereby providing a zone ofrelative quiescence in the lower portion of the cylinder in which, ifdesired', concentrated or solid caustic admixed with liquid ammonia maycollect. The lower portion of this zone may be heated by any convenientdevice, for example, by steam jacket 34 connected to steam lines, 35, orby steam coils, electrical heating elements, etc. Similarly, thereceiver I6 may be equipped with a suitable heating jacket 36,electrical heating element, or other device for purposes of maintainingan elevated temperature of the contents. Receiver I8 is also provided atits top withl a ventf31 for any ammonia vapors which may be entrainedinthe caustic and at its lbottom with a conduit V38 for the discharge ofthe liquid caustic.

In a specific example of the operation of the apparatus described, thecylinder I0 ischarged with liquid ammonia through the conduit 20 andcaustic soda of a concentration of approximately 50 percent or upwardlssupplied through the conduit I8. The ammonia gradually passesvupwardly, taking up water and impurities from the caustic with which `itcontacts.

Baiile plates 28, preferably inclined downwardly from the walls of thecylinder and slightly spaced therefrom, are provided within the cylinderI0 and serve to divide the solution into a multiplicity of superposedrelatively quiescent zones of treatment which are partially segregatedfrom each other. These baffles are large enough to prevent rapidintermixing of the liquor in one zonewith the liquor in another. As aresult of the downward flow of the caustic and the upward Y ow of theliquidammonia, it is readily seen that the composition of the liquor ineach zone will be different from that in any other zone since theammonia will be highly concentrated in the lower zones, andprogressively more dilute in the upper zones. In effect, then, theincoming caustic will be treated by a series of separate ammonia liquorscontaining progressively increasing concentratlons of liquid ammonia,and the more concentrated caustic settling to the lower zones will beintimately contacted with the more concentrated ammonia liquors. Whilebatlies afford a convenient means for establishing this multiplicity ofsuperposed zones it will be apparent 'that other means may be resortedto in order'to obtain the same effect.

Eventually, the diluted ammonia is drawn off at the top as a partiallyhydrated liquor and may be concentrated by distillation for reuse, or itmay be employed to partially purify 50 percent or more concentratedcaustic liquor by removal of NaCl, NaClOa and the like as will behereinafter discussed.

The treated caustic collects in the cylinder below the ammonia inlet. Ifthe temperature is kept above the melting point or the softening pointof the caustic, and the ammonia-caustic ratio is properly maintained, amelted concentrate will be produced. Thus other conditions beingmaintained the same, with increasing temperature the tendency will be toform a molten concentrate, and with lower temperatures, to form a solidconcentrate. With constant temperature,. with an increasing ratio ofammonia to caustic under treatment, the tendency will be in thedirection of a. greater degree of dehydration and with it, to form asolid concentrate, and conversely. If desired, the caustic may be causedto solidify during settling in the lower portion of the cylinder asillustrated. 'Ihe solidified caustic phase may then be liquefied in theconical bottom of the cylinder by the application of heat from steamjacket 34. The pressure in the system may be used to force the liquidcaustic downwardly through the conduit I4 into the receiver I6. Thereceiver is usually maintained at a lower internal pressure than thecylinder I0, and upon admission to it of the molten caustic, most of theresidual ammonia carried by it flashes into vapor, passes off throughvent 31 and is conveyed to a receiver (not shown). Traces of ammoniaremaining in the caustic may be removed by suitable methods as, forexample, by blowing gas through the concentrate or by subjecting it tovacuum treatment. Fluidity of. the caustic in the receiver is maintainedby means of the heating element and from time to time, or continuously,the liquid is drawn oi to storage orto apparatus for further treatment.

The form of apparatus shown in Figure 2 may be employed when it isdesired to produce highly concentrated caustic. It consists of anupright shell or cylinder 40 of relatively strong construction towithstand the pressure of liquid ammonia. Within the cylinder there isan agitator 4I and baffles 42. A conduit 46 disposed. below the upperend of cylinder 40 is provided for admission of. the caustic liquor tobe treated. The cylinder 40 discharges at its lower end into an upwardlyinclined cylinder or leg 43, and is also provided with a conduit 44 atits upper end for the discharge of spent ammonia liquor.

In the inclined leg or cylinder 43, there is an axially disposed shaft48 which at its lower end carries a screw conveyor 50, designed to movethe solids axially up the cylinder and thus to separate them from theammonia and caustic liquor. At the upper end of cylinder 43 a series ofspirally arranged scoops or paddles 5|, carried by shaft 48, areprovided to agitate the solid sodium hydroxide moving up throughcylinder 43 to prevent the particles from agglomerating and to secure amore adequate washing of the solids. Concentrated liquid ammonia isadmitted to the inclined cylinder 43 at a point intermediate the lengththereof through a suitable conduit 52. A suitable steam jacket 53 orother heating element may be provided about the cylinder 43, for'purposes of. assisting in vaporizing ammonia in the upper portionthereof.

The upper extremity of the inclined cylinder 43 discharges into asuitable receiver 54 which preferably is provided with a heating device,for example, a steam jacket 56 connected to steam lines 51. The receiveris also provided at its lower extremity with a discharge conduit 58through which liquid caustic may be permitted to ilow to a suitablereceiver.

In the operation of this embodiment of the invention, caustic sodasolution is admitted through the conduit 46 to cylinder 40, and ammoniain the liquid state, preferably substantially anhydrous, is admittedthrough conduit 52, passing through cylinder 43 to enter the lower endof cylinder 40. As the two liquids pass countercurrently through thecylinder 40, concentration of the caustic phase to 60, 65, 70, or higherpercent may result.

The concentrated product settles from cylinder 4u into the lower end of.cylinder 43 and may be maintained at a temperature sufliciently high toprevent the precipitation of the monohydrate. A temperature of about '70C. will be satisfactory for this purpose. Anhydrous sodium hydroxide iscaused to precipitate from the heated solution by the introduction ofthe substantially anhydrous liquid ammonia at 52 and the precipitatedcaustic is carried upwardly through the cylinder by the screw 50 and isdischarged into the receiver 54. A temperature suciently high tovaporize the ammonia may be maintained in the upper portion of leg 43whereby a vapor zone is created. As the solid anhydrous caustic isremoved, a quantity of caustic liquor will be occluded, absorbed orotherwise carried along by it and the concentration of the final productwill depend upon how much of this liquor is carried aman over. If theconveyor is operated rapidly, or if the cylinder 43 is only slightlyinclined, it will be found that the product drawn oil| at'5 will be of aconcentration somewhat in excess of 70 percent, but will not beanhydrous. On the other hand, if the conveyor operates more slowly, orif the cylinder 43 is more steeply inclined, a more nearly anhydrous ora substantially anhydrous product can be secured since the solid causticwill drain and will be more thoroughly washed by the ammonia. Regulationof the rate of operation of the conveyor, the inclination of thecylinder 43, and control of the proportion oi'. liquid ammonia usedaffords an easy method of regulating the concentration of caustic to beproduced.

By this process, I obtain a very advantageous washing of the anhydrouscaustic since it is removed in a direction counter-currently to theincoming liquid ammonia which bathes the outgoing anhydrous caustic andremoves impurities or absorbed caustic liquor therefrom, or it may alsodehydrate the absorbed caustic liquor. While I have described thismodification of my invention in connection with the production ofanhydrous caustic, it should be noted that it is equally valuable whenany solldied product, such as hydrates or ammoniates of caustic, orcaustic soda of any other concentration is being formed under similarcircumstances.

The ammonia passing off from column 40 may be passed directly to asuitable ammonia still (not shown) for reconcentration, or it may bepassed to a second column 60 for a purpose and in a manner to behereinafter described.

The caustic deposited in receiver 54 may be melted and withdrawn at 58,or may be removed as a solid. A liquid concentrate in the neighborhoodor about '70 percent may be Withdrawn from the lower end of cylinder 43as at 10. In each case it is found that the resultant concentrate willbe of high purity and substantially uncontaminated. It appears that theammonia containing more or less water withdrawn from the causticexercises a preferential solvent effect for impurities such as sodiumchloride. sodium chlorate, and others. No substantial contaminationoccurs during dehydration, and I am able to produce anhydrous, or otherconcentrations of caustic, which are just as pure, or in some cases evenpurer than the 50 percent concentrate from which it is derived. Thisprocess is especially adapted for the treatment of electrolytic caustic,since by operating in accordance with this process I am able to produceelectrolytic caustic of even higher purity than that produced by theammonia-soda process. -While I have described this process in relationto the production of caustic in concentrations in excess of 70 percent,it is evident that it may be used to produce caustic in'anyconcentrations above 50 percent and up to about 100 per cent. Forexample, the dihydrate might be held in solution to permit themonohydrate to precipitate and the concentration of the product producedcan then be regulated by regulating the amount of solution drawn offwith the precipitated monohydrate.

Various modifications of the above processes are permissible as eithersolid or liquid aqueous caustic may be concentrated to produce a solidor liquid product. Thus, I may introduce solid or liquid caustic at thetop of cylinders l0 and 40, respectively, and maintain the causticduring the course of treatment either in the solid or liquid state; or.I may permit the caustic to `precipitate or solidify during thetreatment. I'he caustic, after treatment, may be withdrawn as a liquidor a solid. Usually, I find it preferable to introduce the caustic inthe liquid state and maintain" it as a liquid during the course of thetreatment and remove it asa liquid from the reactor; or, I may permitthe treated liquid caustic to solidify or crystallize near the bottom ofthe reactor and remove it as a crystallized product or remelt thecrystals or solid and remove the caustic in the liquid state. The abovevariations may be controlled by the operating conditions such astemperature, pressure, agitation, contact time, etc., and by control ofthe concentration of the caustic and ammonia, as will be more fullydescribed subsequently.

The temperature maintained during treatment is capable of widevariation, so long as it is below the critical temperature of ammonia,being largely dependent upon whether solid or liquid concentrates are tobe produced. It is not necessary that a uniform temperature bemaintained throughout the apparatus, and very often it is desirable thatthe temperature should not be uniform. Thus, in the columns I0 and 40,respectively, the temperature may vary from top to bottom so thatentering caustic is subjected to a gradually increasing or decreasingtemperature. Again, in some cases, it may be desirable to subject thecaustic to repeated solidication and melting treatments as it passesdownwardly through the cylinder, thereby securing an additionalpurification by the crystallization of a pur'er caustic out of an impuremother liquor. Under such conditions, the caustic moving downwardlywould be passed through alternate zones of higher and lowertemperatures.

The pressures=must, of course, be suflicient to maintain the ammonia inthe zone of dehydration in the. liquid state. In general, the pressuremay be -maintained at some value upwards of 100 pounds per square inch.The degree of agitation and time of contact should be such that theliquid ammonia and the caustic being treated are intimately commingledand will depend largely upon the degree of concentration desired andwhether it is desired to secure a liquid or solid product.

' in, the two liquid phases may tend to merge and nally disappear. Ingeneral, the process is controlled in such a manner as to enable thewithdrawal of an ammonia liquor containing upwards of 65 percent NH3.The tendency of the phases to disappear is especially noticeable whensolutions of low concentration (20-30 percent) are introduced in theupper portion of the column. In treating solutions of highconcentration, for example, about 50 percent or above no greatdifliculty is encountered in the ensuing separation of two liquidphases.

When the treatment is conducted countercurrently with vigorous agitationin a column at a temperature of about '10 C. or thereabout, a ratio ofabout one to two parts of anhydrous liquid ammonia to one part of 50percent caustic liquor is usually sulcient, but this ratio may likewisebe widely varied.

Suitable illustrations of the process as carried out in the apparatus ofFigures 1 and 2 are given as follows:

'In one electrolytic caustic liquor having the following analysis- Percent NaOH (approx.) l 50 Nafl 1.05 Noam. 0.46

was continuously introduced at 1111er la in the apparatus of Fig. 1, andvsubstantially anhydrous liquid ammonia continuously introduced at inlet.'20 in proportion of one and one-half parts by Per cent NaOH (approx.)70 NaCl 0.06 NaClO3 0.00

In a similar manner, one part of 50 percent, (approxJ, caustic of Athecharacter above described was continuously treated with ve parts ofliquid ammonia at atemperature of '10 C. and a pressure of '150 poundsper square inch, using the device shown inY Fig. 2. In this case,precipitated caustic was withdrawn from the base of the column 40,further treated in column 43, and deposited in receiver 54, melted at atemperature of 317 C. and withdrawn. The concentration of caustic wasapproximately 98 percent, and the percentage of impurities was markedlyreduced, as in the previous example.

The flow sheet, shown in Fig. 3, illustrates another modification of myinvention. It is to be understood that this flow sheet is merely adiagrammatic illustration and that many other modifications arepossible. A three-stage process of dehydratingcaustic is hereinillustrated wherein water is progressively removed in each of the threetreaters, 1 I, 12, and 13. These treaters may be of any suitableconstruction, as for example, those described in Figs. 1 and 2. This isin no way essential, however, and it is to be understood that anyconvenient apparatus or type of settling tank may be used in whichseparation into a. caustic phase and a liquidammonia phase is secured.

In the specic embodiment shown, a 50 percent caustic .solution isintroduced into treater 1I at 14, and fresh liquid ammonia is introducedinto treater 13 at inlet 15. The caustic in treater 1I is subjected towater absorption by contact with somewhat diluted liquid ammonia derivedfrom treater 12 through conduit 19. This treatment concentratesthe-caustic solution and the separated concentrated caustic phase isdrawn off at 16. The further diluted ammonia liquor is removed at 8| andmay be reconcentrated and recycled or it may be used to treat othercaustic liquors for dehydration and/or puriiication.

This process is repeated in treater 12 wherein the caustic enteringthrough conduit 16 is treated with partially hydrated liquid ammoniaentering through conduit 11, a still more concentrated caustic beingwithdrawn through conduit 18 and given a final concentration by thefresh liquid ammonia entering through conduit 15 in treater 13.

Concentration in each stage may be countercurrent, asin Figs. 1 and 2,or I may concentrate in each stage by one or a multiplicity of batch thepresent invention may be-produced by any of the processes which havebeen accepted comf treatments or in any other desired manner. Thepercent caustic to be treated bythe process of hydrated ammonia liquormay be introduced into the last stage. 'I'he following is an example ofthe process:

50 percent caustic was treated countercurrent- 1y in'two stages,substantially anhydrous liquid ammonia being introduced in the finalstage. In the rst stage, the caustic was concentrated to '10 percent bytreatment with the partially hydrated ammonia liquor fromthe secondstage, removed and allowed to solidify. This 'product was re moved to asecond stage treatment, in which it was melted and treated withsubstantially anhydrous ammonia. A.v highly concentrated. iiuy, porouscaustic was produced in good yield and with high purity. 'Ihe ammonialiquor from the second stage treatment was used as the concentratingliquor in the first stage. The temperature of the rst stage was C., andin the second stage was C. The concentrate produced was extremely pureand no contamination occurred during treatment.

In a second example, 59 percent caustic was treated countercurrently in3 stages with liquid ammonia, winch as introduced in the nal stage,contained about 5 percent water. In the ilrst stage, the caustic wasconcentrated to 65 percent by treatment with ammonia liquor from'thesecond stage, removed in a melted state and introduced into the secondstage. In the second stage, by treatment of the ammonia liquor from thefinal stage, the caustic was concentrated to percent and was then sentto the third stage from which a caustic of about 83 percentconcentration was withdrawn as a melted product, and allowed tosolidify. A temperature of '75 C. was maintained in the three stages.The product was extremely pure and no contamination occurred duringtreatment.

In carrying out the invention, I may also treat a caustic liquor of anydesired concentration, for example, 59 percent, with anhydrous liquidammonia and use the resultant ammonia to treat more caustic of thesame'or diierent concentration, or to purify caustic without substantialconcentration. i

For example, in a ilnal stage of treatment, as in treater 13 in theapparatus of Fig. 3, one part of 70 to '71 percent caustic was treatedwith rive parts of anhydrous ammonia at a temperature of 70 C.v A solidcaustic produced was crystallized and withdrawn and two parts of theremaining ammonia liquor were used in treater 'l2 to treat one part ofabout 60 percent caustic derived from treater 1I, at a temperature of'10 C. Partially dehydrated caustic of about 71 percent was withdrawn,passing into treater 'I3 and the ammonia liquor again used to partiallytic, the concentration to be obtained in the nal dehydrate and purify athird portion of caustic of 5l percent concentration in treater 1|.

While from a commercial standpoint, it may be desirable to make 'use `ofsome process involving counter-current treatment, it may often be adlyvisable to progessively remove water by. successive washing vwithseparate portions of liquid ammonia. 'Ihe following examples illustratethis phase of my invention. l

Example 1,-About parts by weight of 50 w percent NaOH at 100 degrees C.was treated in an autoclave successively with three portions ofanhydrous liquid ammonia having a combined weight of 300 parts. Afterthe last extraction, the residue consisted of approximately 50 parts ofl5 anhydrous. crystalline, free-flowing caustic soda. 'I'he total amountof ammonia employed may be varied over a relatively wide range dependingupon the initial concentration of the caustic solution. As little asabout two parts by weight zo per part of anhydrous` caustic obtained maybe suiilcient. On the other hand, ten or more parts of ammonia per partof anhydrous caustic are also feasible.

Example 2.--About Aparts by weight of 50 25 percent NaOH at 80 C. weretreated successively with five portions of anhydrous ammonia having acombined weight equal to about eight times that ofthe anhydrous NaOH.The original solution contained 2.28% NaCl on solid basis (a typi- 30cal electrolytic caustic liquor), while the residue after treatment wasanhydrous. crsytalline caustic containing only .03% NaCl.

Example 3.--About 200 parts by weight of 'I2 percent NaOH were treatedwith successive portions of liquid ammonia at 50 C. amounting in all to350 parts. The water was progressively absorbed from the caustic witheventual formation of crystals of anhydrous material. The resultinganhydrous caustic was an exceedingly g light porous uny powder; thatisto say, a powder, the individual particles of which contain ilne poresor cells..

Eampte About two parts by weight of 50 percent electrolytic NaOHcontaining 0.43 peru, cent chlorate and 1.02 percent sodium chloride,was treated at '10 C. in iron equipmenhunder a pressure of '100 poundsper square inch, with three vparts by weight of anhydrous ammonia andthe ammonia liquor removed. 'I'he resultant product was subjected tosuccessive washings with liquid ammonia in the same manner until aresidue of anhydrous crystalline caustic was produced. No chlorate wasfound in the product, and the sodium chloride content was'found to be0.04 percent.

The number of treatments required toremove the water from theconcentrate will be governed in part by the concentration of the initialcausproduct, the concentration of liquid ammonia 6 to occur in the laterstages oi' the process, but 70 I iind that the major portions of theconcentration is best eiIected while the caustic is kept molten. 4Thistends to minimize any tendency of the device to plug and thereforebecome inoperative. However, if it is desired, I can'conu centratecaustic either in the solid or liquid state; and, by proper regulationof the temperature, pressure, agitation, con'tact andv concentration ofammonia used, either solid anhydrous, or hydrated caustic may beprecipitated. Inaccordance with one phase of my invention, I may addsufficient ammonia to caustic liquor to cause separation of two liquidphases and a solid caustic phase. The solid precipitate vmay then beremoved in a suitable manner, forexample, 'by'flltration By one veryeective method of filtration, the liquor may be filtered in such amanner that the lower phase is first withdrawn' through the filter, theammonia liquor phase passing through the 4filter thereafter. Bysuch aprocess, I am able to secure a desirable washing of the solid hydroxidewhich collectsupon the iilter surface by the ammonia liquor therebyassuring the production of very pure hydrated caustic. l

Inv some cases, concentration of solid hydrated caustic without anymelting whatsoever may be desirable. kFor example, I have found thatsolid hydrated caustic will give up part or all of its water whenextracted with an hydrous or par- `tiallv hydrated liquid ammonia. -Thisaffords a very simple method for concentrating solidified concentratesvwhich have been produced by any process, as itgmay be carried out byfinely divlding. the concentrate, intimately' mixing the` liquidammonia, and removing the ammonia with its absorbed water.

I am not limited to treatment of form, however, and the production ofconcentrates from either liquid or solid caustic is within the scope ofmy invention. Thus, I may produce both a 'liquid concentrate anda solidhydrate simultaneously, or I may find it desirable,'in order to secure aparticularly pure product, to convert the material to liquid, and thencrystalline a solid and remelt, recrystallize, etc., until the desiredApurity and concentration is attained.

Attention is now directed to the modication of my processdiagrammatically illustrated in Figure 2. I have discovered that inoperating in accordance with myv invention, if caustic liquor of an NaOHconcentration of 50 percent or higher is treated with an ammonia liquorcontainingA water in such proportions that, on admixture, two phasesform in which the proportions of water are respectively the same as inthe original liquors (the liquors being substantially in equilibrium asto their water content), the ammonia phase, being the lighter, will formthe upper layer and the lower layer will be the caustic phase of aboutthe same concentration, (on the ammonia free basis) as the originalcaustic liquor. When caustic liquor such as is derived from theelectrolysis of sodium chloride, containing sodium chloride, chlorates,and other impurities evolved during' caustic production, is contactedwith ammonia in this manner, the impurities distribute themselvesbetween the two phases. The distribution ratio is such that the minorportion of these impurities is left in the caustic phase and in the caseof sodium chlorate, substantially none. For example, if a 50 percentcaustic liquor is thus treated with ammonia liquor containing about "10percent liquid ammonia and 30 percent water, the resulting puriiiedcaustic liquor is of about 50 percent concentration.

The caustic liquor will not be substantially concentrated or dilutedunder these conditions caustic in any since liquid ammonia of the abovedilution is in substantial equilibrium with a bpercent caustic solution,and will absorbl substantially no water therefrom.

YBy this method, I am able to treat an already fairly pure solution ofcaustic and to remove certain compounds, such as sodium chloride, sodiumchlorate,v and others which up until now have not been satisfactorilydisposed of, thereby pro- 'ducing a similar concentrate which is purerthan has hitherto been considered economically feasible. y

In this manner, I:` am able to secure purication of caustic soda withoutsubstantial concentration in a. simple manner since the caustic may betreated with ammonia, -or ammonia-water mixtures of such water contentthat no substanvtial water is absorbed from the caustic. "I'hus,

if anhydrous caustic soda is to be purified, an-

hydrous liquid ammonia would be used while if hydrated caustic is to bepurified, it should be treatedv with ammonia liquor containing the'pre-`determined quantity of water necessary to preumn 40 to Fig. 2 isconducted through conduit M into the purifyingcolumn 60 at a-pointsomewhat above the bottom thereof. 'Ine column is provided with aninlet' 62 for the caustic to be purified and an outlet B6 for theammoniacal liquor. The bottorn'portion BI of column 60 is conical and,at the apex discharges 4through an outlet 68 for caustic liquors.Column'il) may be provided with suitable agitator and baille structuressubstantially similar to those in column 40. The caustic, in beingbrought into contact with the diluted ammonia liquor as described above,isfreed from its impurities andAv is discharged from the apparatus.. Theconcentration of the puriiied caustic is not substantially increasedover that of the untreated caustic and this process can be controlled toproduce a purified caustic of a concentration which is substantially thesame as that of the impure caustic in a manner more fully described andclaimed in my copending application (Case A-Z'l) Serial No. 179,335, ledDecember 11, 1937. In accordance with the `vention, the diluted ammonialiquor from coly invention claimed herein, the operation in colsuch adesired water content that it can be used in column 60 to purify 50percent caustic without substantial concentration thereof. The purifiedcaustic liquor from tower'60 may also be used in the concentratingoperation in columns 40 and 43 or in column 40 alone, thus avoidingexcessive contamination of the ammonia liquor or liquid ammonia used inthe process.

While my process has been described with reference to caustic-sodatreatment, it may be similarly applied to the purification andconcentration o any alkali metalA hydroxide such as lithium andpotassium hydroxides. Furthermore, it is not necessary that liquidammonia be used since gaseous ammonia may be added and liquened duringthe operation. It is also within` the purview of my disclosure to usesubstituted ammonias such as methylamine, dimethylamine, andtrimethylamine, Although the present invention has been described inconnection with the specic details of.

certain embodiments thereof. it is not intended that such details shallbe regarded as limitations upon the scope of the invention exceptinsofar as included in the accompanying claims.

By the terms hydrated alkali metal lnrdrox- I ide, hydrated sodiumhydroxide," etc., in the i following claims, I intend to include the useof both aqueous solutions and solid hydroxides containing Water.' Y 1 1.In a process of concentrating a sodium hydroxide-water mixture to aconcentration substantially in excess of per cent, the stepl whichcomprises subjectinga sodium hydroxide-water mixture of at least about 50 Vpercent initial conl centration to progressive washing with partiallyhydrated liquid ammonia containing progressively increasingconcentrations of ammonia the amount of ammonia present during treatmentbeing suillcient'to insure the existence cfa liquid phase containinga-major portionof the ammonia and a second phase containing a majorportion ofsaid hydroxide.

2. In 'a process of. concentrating an aqueous solution of sodiumhydroxide to a concentration substantially in excess of 5 0 percent, thestep which comprises subjecting an aqueous solution A of sodiumhydroxide of at least about 50 percent initial sodium hydroxideconcentrationto treatnient with the quantity of a material of the groupconsisting of liquid ammonia and mixtures lof liquid ammonia Aand waterrequired to absorb sufilcient water to'yield an alkali hydroxide of aconcentration of at least 60 percent of sodium hydroxide and removingthe said ammonia and absorbed water the'amount of ammonia present duringtreatment Vbeing sufcient to insure the existence of a liquid phasecontaining a major portion of the ammonia and a second liquid phasecontaining a substantial portion of said hydroxide.

`3. In a process of concentrating a sodium hydroxide-Water mixture toaconcentration substantially in excess of 50 percent, the step whichcomprises subjecting a sodium hydroxide-water mixture of at least about50 percent concentration to treatment with a material of the 'groupconsistingof liquid ammonia and mixtures of liquid ammonia and water inAcountercurrent ilow the amount of ammonia present during treatmentbeing sufficient to insure theexistence of a liquid phase containing amajor portion of the ammonia and a second phase containing a majorportion of said hydroxide.

4. The process of concentrating hydrated sodium hydroxide to secure aconcentration substantially in excess of 50 percent, which comprisessubjecting a sodium hydroxide-water mixture of at least 50 percentsodium hydroxide concentration to treatment with -a material of thegroup consisting ofliquld ammonia and mixtures of liquid ammonia andWater in .countercurrent ilow; whereby water is withdrawn from saidhydroxide, and withdrawing concentrated sodium hydroxide the amount ofammonia present during treatment beingv sufcient to insure the existenceof a liquid phase .containing Aa major portion of the ammonia and asecond phasecontaining a major and water. and maintainingthe-temperature of operation under the pressure obtaining, sumcientlyhigh and' below the critical temperature of ammonia to preventsubstantial solidiilcation ofthe resultant concentrate at least duringthe major portion of the treatment the amount of ammonia present duringtreatment being suillcient to insure the existence of a liquid phasecontaining a major portion of the ammonia and a second phase containinga major'portion of said hydroxide.

6. In a processor concentrating aqueous alkali metal hydroxide to aconcentration substantially above 50 percent, .the step which comprisessubiecting the aqueous alkali metal hydroxide of a concentration aboveabout 50 percent to washing with liquid ammonia the amount of ammoniapresent during treatment being sumcient vto insure the existence of aliquid phase containing a major portion of the ammonia and a secondliquid phasecontaining a substantial portion of said hydroxide.

4'?. A process of concentrating hydrated sodi.-

` um hydroxide which comprises establishing a zone of treatmentintroducing an upwardly owing body' of a material of the groupconsisting of liquid ammonia and mixtures of liquid ammonia andv waterinto a lower portion of said zone and aqueous sodium hydroxide of aconcentration not substantially less than about 50 percent into theupper portion of `said zone; permitting the hydroxide to settle throughsaid zone with consequent concentration due to Water absorption actionof said material, withdrawing concentrated sodium hydroxide from a lowerportion of saidzone and dilutedliquid ammonia from an upper portion ofsaid zone the amount of ammonia present during treatment beingsufllcient to insure the existence of a liquid phase containing a majorportion of the ammonia and a second phase contain.

ing a major portion of said hydroxide.

8. A process of concentrating hydrated sodium hydroxide which comprisesestablishing a zone of treatment, introducing an upwardly flowing bodyof a material of the group consisting of liquid ammonia and mixtures ofliquid ammonia and water into a lower portion of said zone and aqueoussodium hydroxide of a concentration not substantially less than about 50percent into the upper` portion of said zone; permitting the hydroxideto settle through said zone with consequent concentration due to waterabsorptionaction of said material, and maintaining .the conditions ofoperation such that substantial solldication is prevented at least whilethe major portion of the concentration is being effected the amount ofammonia present during treatment being suihcient to insure the existenceof a liquid phase containing a maior portion of the ammonia and a secondphase containing a maior portion of said hydroxide.

9. A process of concentrating hydrated sodium hydroxide which comprises,establishing `a zone of treatment, introducing an upwardly flowing bodyof a material of the group consisting of liquid ammonia and mixtures ofliquid ammonia and water into a lower portion of said zone and aqueoussodium hydroxide of a concentration not substantially less than about 50percent into the upper portion of said zone; permitting the hyldroxideto settle through said zone with consequent concentration due to waterabsorption action of said material, and maintaining temperature,pressure, and concentration of ammonia and sodium `hydroxide such that asolidied concenlower levels, introducing the hydroxide to be treatedinto the uppermost zone and a material of the group consisting of liquidammonia and mixtures of liquid ammonia and water into the lowest zonewhereby the two materials flow from zone to zone countercurrently `toeach other, in timately mixing the liquor in each zone while maintaininga substantial segregation of the liquor in each zone from the liquors inother zones, and withdrawing concentrated alkali metal hydroxide from alower zone the amount of ammonia present during treatment beingsufilcient to insure the existence of a liquid phase containing a majorportion of the ammonia and a second .phase containing a maior portion ofsaid hydroxide.

11. A process of concentrating hydrated sodium hydroxide which comprisesestablishing a plurality of zones of treatment at progressively lowerlevels, introducing the hydroxide to be treated into the uppermost zoneand a material of the group consisting of liquid ammonia and mixtures ofliquid ammonia and water into the lowest zone whereby the two materialsow from zone to zone countercurrently to each other, intimately mixingthe liquor in each zone while maintaining a substantial segregation ofthe liquor in each zone from the liquors in other zones and withdrawingconcentrated sodium hydroxide from a lower zone the amount of ammoniapresent during treatment do being sufficient to insure the existence ofa liquid phase containing a major portion of ythe ammonia and a secondphase containing a maior portion of said hydroxide. v

12. A method of concentrating hydrated alkali metal hydroxide whichcomprises establishing a body comprising aqueous alkali metal hydroxidehaving a concentration above about 50 percent hydroxide, introducing a,material of the group consisting of liquid ammonia and mixtures ofliquid ammonia and water into said body, and controlling the conditionsof temperature, concentration of ammonia, and concentration of causticsuch that a solid product will be precipitated and withdrawing saidsolid product countercurrent to the incoming ammonia liquor to eifectWashing thereby the amount of ammonia present during treatment beingsuflicient to insure the existence of a liquid phase containing a majorportion of the ammonia and a second phase containing a major portion ofsaid hydroxide.

13. The process of concentrating hydrated sodium hydroxide whichcomprises contacting a material of the group consisting of liquidammonia and mixtures of liquid ammonia and water with solid hydratedsodium hydroxide while maintaining the conditions of operation such thata maior portion of said hydroxide remains in the solid state throughoutthe process and separating the solid concentrated sodium hydroxide theamount of ammonia present during treatment being suiiicient to insurethe existence' of a liquid phase containing a major portion of thearnmonia and a second phase containing a maior portion of saidhydroxide.

14. A process for concentrating and purifying of treatment atprogressivelyA solid hydrated sodium hydroxide which comprisescontacting the said sodium hydroxide in solid state with a material ofthe group consisting oi' liquid ammonia and mixtures of liquid ammoniaand water to thereby produce a hydroxide of greater concentration theamount oi 'ammonia present during treatment being suilicient to insurethe existence of a liquid phase containing a maior portion'of theammonia and a second phase containing a major portion of said hydroxide.

15. The process oi concentrating hydrated sodium hydroxide whichcomprises establishing an upwardly flowing body comprising a material ofthe group consisting of liquid ammonia and mixtures of liquid ammoniaand water, introducing solid hydrated sodium hydroxide of aconcentration not substantially less than about 50 percent into saidbody to permit settling therethru and withdrawing concentrated sodiumhydroxide from a lower portion of said stream the amount of ammoniapresent during treatment being sufficient to insure the existence of aliquid phase containing a major portion of the ammonia and a secondphase containing a maior portion of said hydroxide.

16. The method of concentrating sodiimi hydroxide which comprisescountercurrently treating hydrated sodium hydroxide with a material ofthe group consisting of liquid ammonia and mixtures of liquid ammoniaand water in such concentration and at such temperature as to producesolidified hydroxide and subjecting said solidied hydroxide to a washingtreatment by withdrawing it countercurrent to the ammonia liquorintroduced into the process the amount of ammonia present duringtreatment being suilieient to insure the existence of a liquid phasecontaining a major portion of the ammonia and a second phase containinga major portion oi' said hydroxide.

17. The method of concentrating hydrated sodium hydroxide whichcomprises establishing a zone of treatment comprising an upwardlyilowingbody of a material of the group consisting of liquid ammonia andmixtures oi liquid ammonia and water, introducing hydrated sodiumhydroxide into said zone to settle thru said owing stream withconsequent concentration, collecting the resultant concentratedhydroxide in the lower portion of said zone, introducing fresh liquidammonia into said zone whereby a substantial precipitate of solidhydroxide concentrate is formed `rand withdrawing said precipitatecountercurrently of the incoming ammonia to effect washing thereby theamount of ammonia present during treatment being sumcient to insure theexistence of a liquid phase containing a maior portion of the ammoniaand a second phase containing a major portion of said hydroxide.

18. A method of purifying hydrated alkali metal hydroxides whichcomprises treating said hydroxides with a material of the groupconsisting of liquid ammonia and mixtures of liquid ammonia and water insuch concentration and at such conditions of temperature that solidhydroxide separates from a liquid phase consisting preponderantly ofammonia and water, and a second liquid phase consisting preponderantlyof alkali metal hydroxide and water, and ltering the entire liquid bodyso formed in a manner such that the ilrst named liquid phase passesthrough the lter during the later stages of iiltration, whereby solidmaterial deposited upon the filter surface is washed by the ammonialiquor of said mst-named phase the amount of ammonia sure the existenceof a liquid phase containing a major portion of the ammonia and a.second phase containing a major portion of said hydroxide.

19. A, method of concentrating aqueous caustic soda of not substantiallyless than 50 percent initial concentration which comprises causing thecaustic to contact withsubstantially anhydrous liquid ammonia incountercurrent iiow until the caustic is converted by absorption ofwater to a product consisting largely of the monohydrate containingabout 69 percent of sodium hydroxide.

20. A continuous process for concentrating an hydrated alkali metalhydroxide which comprises establishing a body containing an aqueousalkali metal hydroxide having a concentration not substantially lessthan 59 percent by weight. calculated upon the ammonia-free basis,introducing a material of the group consisting-of liquid am- 'monia 'andmixtures of liquid ammonia and water into a lower portion of said bodyin an amount suicient to insure the separation of a, liquid phasecontaining the major portion of the ammonia from a second liquid phasecontaining a major portion of said hydroxide, introducing aqueous alkalimetal hydroxide to be concentrated into an upper portion of said bodyand withdrawing more concentrated hydroxide from a lower portion of saidbody.

21. A process for dehydrating an aqueous alkali metal hydroxide whichcomprises countercurrently contacting said hydroxide at a progressivelyincreasing temperature with a material 'of the group consistingl ofliquid ammonia and mixtures of liquid ammonia and water, the amount ofliquid ammonia being suiiicient to insure the separation of a liquidphase containing a major porl tion of the ammonia from a liquid phasecontaining a portion of said hydroxide.

22. 'A process of concentrating hydrated sodium hydroxide whichcomprises treating said hydroxide with a material of the groupconsisting of liquid ammonia and mixtures of liquid ammonia and water,the amount of ammonia present during treatment being sufficient toinsure the existence of a liquid phase containing a major portion of theammonia and a quantity of water absorbed from said hydroxide, a secondliquid phase containing a substantial portion of said hydroxide and asolidified hydroxide of higher concentration than the hyroxide in saidliquid phase and removing solidifled hydroxide together with some ofsaid second liquid phase.

23. The process of claim 22 wherein the temperature is maintainedsulciently high to prevent precipitation of a solid hydrate whereby thesolidiiied hydroxide is substantially anhydrous hydroxide.

24. A process of concentrating hydrated` sodium hydroxide whichcomprises treating said hydroxide with a material of the groupconsisting oi liquid ammonia and mixtures of liquid ammonia and water,the amount of ammonia present during treatment being'sumcient to insurethe existence of a liquid phase containing a major portion of theammonia and a quantity of water absorbed from said hydroxide, a secondliquid phase containing a substantial portion of said hydroxide and asolidified hydroxide of higher concentration than the hydroxide in saidliquid' phase and removing solidied hydroxide.

SERVING E. MUSKAT.

